The invention relates to hydrated fibers predominantly consisting of water glass, processes for their production by the dry-spinning principle and further processing into silica fibers, silica fibers as such, and their utilization, especially in friction linings as used for instance in automatic transmissions, as lining for clutches and brakes etc.
Water glass fibers are in the category of inorganic fibers, and have been known, per se, for some time. The British Pat. No. 352 681 note a number of inorganic fibers that may be obtained from different silicates, wherein the silicates contain, apart from the alkaline metals also other metals such as aluminum, magnesium etc. A number of processes for the production of such fibers have been noted, for instance the wet spinning process. Without futher information on the composition of the spinning stock and the spinning conditions, the so-called dry spinning process is also made mention of.
In the U.S. Pat. No. 2,338,463, the production of, respectively, silica fibers or quartz-glass fibers is described. Water-glass fibers will result herein as intermediate product which may be produced by various processes such as the melt extrusion process, the wet spinning process or the dry spinning process. Spinning of a so-called double water glass by the dry spinning process is described in example 2 of these Letters Patent, the double water glass being a water glass containing equimolar quantities of K.sub.2 O and Na.sub.2 O.
In after-treatment relating to the example, it has, however, become evident that the composition described therein is solid at room temperature and thus cannot be spun. If the temperature of the stock is raised, so that it becomes extrudable, considerable difficulties will result at the spinning die. Lumps or droplets will form and clog the discharge side of the die within a short time.
In U.S. Pat. No. 2,969,272 also describe a further process for the production of inorganic fibers by dry spinning of silicates from aqueous solutions. In the process described therein, it is of disadvantage that the silicates used contain additional metallic oxides which may be causing defects during subsequent treatment. With the process as per U.S. Pat. No. 2,969,272, relatively low limits are also imposed upon the spinning velocity.
Although a number of remarks relating to the production of water glass fibers by the dry spinning process are already present in literature, the need exists for improved production processes that will be defect-prone to a lesser extent and yield fibers with improved properties.
It is therefore the task of the invention to make available a process less defect-prone than the processes known hitherto and allowing a high spinning velocity. It is furthermore the task of the invention to enable the production of water glass fibers using aqueous solutions for spinning which are free from non-alkaline metallic compounds such as compounds of aluminum, boron, magnesium, zinc, calcium etc., and with which it will not be necessary to add such compounds, as noted in the U.S. Pat. No. 2,969,272, in order to make the stock suitable for spinning.
It is furthermore the task of the invention, to make available hydrated water glass fibers of good mechanical properties such as high tear strength and a favorable modulus of elasticity, which can be processed into very pure silica fibers and which will be suitable as reinforcing inserts for various materials. And it is furthermore a task of the invention to make available water glass fibers that may readily be reeled up after spinning and that may, without any further measures, be treated with preparations of various types as needed for further treatment.
Fibers on the basis of silicon dioxide or silica respectively, containing, apart from silicon, oxygen, and smaller quantities of water, with practically no other constituents such as metallic compounds, are to be understood as silica fibers within the purview of the present invention.
Silica fibers and silicon dioxide fibers respectively, have been known for a long time. They may be produced, f.i., by spinning of a melt containing SiO.sub.2. Since a process of this type will require temperatures from approximately 2000.degree.-2100.degree. C., special high-temperature resistant apparatus is required and the technical expenditure will be high so that the prices for such fibers will be relatively high.
Glass fibers as for instance described in the DE Letters of Disclosure No. 2 609 419 may also be leached with acids. This process is very cumbersome and labor-intensive, the long leaching times detracting from the mechanical properties and it is, furthermore, difficult to quantitatively remove the cationic constituents.
Filaments have already been spun from solutions such as solutions of cellulosic xanthogenates containing dissolved sodium silicate, as can be seen from FR Pat. No. 1 364 238, or hydrolized tetra-alkoxy silicones in the presence of polyethylene dioxide, as described in the DE Letter of Disclosure No. 2 041 321. It is of disadvantage with processes of this type that, inter alia, they are working with an organic auxiliary substance which becomes lost during a protracted process of pyrolysis thus leading to a considerable increase in production cost.
The GB Pat. No. 352 681 disclose dry-spinning water glass and other soluble silicates, with subsequent treatment in various baths containing acetone, salts or acids. Concrete instructions for the process, especially how to obtain, at first, water glass fibers suitable for subsequent treatment, and pure SiO.sub.2 fibers thereupon, are, however, lacking herein.
Thus the need still exists for processes by which pure silica fibers with valuable properties may be produced from water glass in a simple and advantageous manner.
It is therefore a further task of the invention, to make available a process by which silica fibers with good mechanical properties and essentially free from non-alkaline metallic compounds and with manifold uses, may be produced in a simple manner from water glass fibers obtained with the use of commercial-grade water glass. A process, in which the aforenamed disadvantages will not be present, is a further task of the invention.
Friction linings of the type as named afore, should have a comprehensive number of properties. It is, for instance, required that the friction lining will be heat-resistant to a high degree, since, when used as brake lining, it will have to absorb braking energy with temperatures up to 1100.degree. C. developing therein. It should be highly resistant to abrasion in order to have a high life of the friction linings.
Furthermore, the friction lining must be readily workable so that it may be brought into the desired shape without any difficulties. Finally it is often also desirable that it will allow roughening on one or more sides so that, in given instances, the material may be attached to a substrate such as, for instance, a metallic support by means of a bonding agent.
A large number of friction linings have already become known, having the aforenoted properties to a more or less satisfactory extent. Yet, the need still exists for improved friction linings that can be produced in a simple manner and that may be used in manifold applications.
In most of the known friction linings, asbestos is used as the fibrous constituent, but due to reasons of environmental protection, considerable concern has been voiced recently, directed against its use; asbestos is said to be inimical to health and to be causing or promoting cancer in particular. Legislative measures are therefore expected having the objective of either fully prohibiting the use of asbestos within a foreseeable time, or to preclude it to a far-reaching extent. Entirely apart from this, asbestos is a resource not available in unlimited quantities.
Thus, the need continues to exist for friction linings in which asbestos will no longer be the fibrous constituent.
It is therefore a further task of the invention, to make available a friction lining composed without the hitherto usual asbestos fibers, which may be fabricated in a simple manner and which has good properties of use. A further task of the invention is a friction lining having good thermal stability, in which the reinforcing fibers used have a good bond with the imbedded material, having good abrasion resistance, being distinguished by a particularly homogeneous construction, of a long life, and which, when used as brake lining will not cause the usual screeching noises. After a further task of the invention is a friction lining of manifold applications with a low aggressiveness towards the counter material and having a high friction number with coefficients of dynamic and static friction that are of a favorable ratio relative to each other.